25 June 1950 North Korean army invades South Korea

At approximately 4 a.m. (Korean Standard Time) on a rainy Sunday morning Democratic People's Republic of Korea Army (DPRK - North Korea) artillery and mortars open fire on Republic of Korea (ROK - South Korea) Army positions south of the 38th Parallel, the line then serving as the border between the two countries. The opening barrage is followed shortly by tank/infantry attacks at all points along the Parallel. At 11 a.m. North Korea announced a formal declaration of war and what is now known as "The Korean War" officially began. In this announcement North Korea claimed ROK forces on the Ongjin Peninsula attacked North Korea in the Haeju area (west) and their declaration of war was in response to this attack. This claim was bogus.

The United Nations Security Council was convened in a few hours and passed the UNSC Resolution 82 condemning the North Korean aggression unanimously. The resolution was adopted mainly because the Soviet Union, a veto-wielding power, had been boycotting proceedings since January, in protest that the Republic of China (Taiwan) and not the People's Republic of China held a permanent seat on the council.[47] President Truman had made a statement on June 27, 1950 ordering the United States air and sea forces to give the South Korean regime support. While the United Nations Security Council was convened and had been debating the issue from the invasion forward it only issued Resolution 83 on June 27 which definitively recommended member-states militarily assist the Republic of Korea. The Soviet Union's foreign minister accused the United States of starting armed intervention on behalf of the Republic of Korea before the Security Council was summoned to meet on June 27, and confronting the UN with a fait accompli.

After the first two months of war, South Korean Army (ROKA) and the US forces rapidly dispatched to Korea were on the point of defeat. As a result, the ROKA and US troops retreated to a small area behind a defensive line known as the Pusan Perimeter.

In September 1950, an amphibious UN counter-offensive was launched at Incheon, and cut off many KPA troops in South Korea. Those who escaped envelopment and capture were forced back north. UN forces invaded North Korea in October 1950 and moved rapidly towards the Yalu River—the border with China—but on 19 October 1950, Chinese forces of the People's Volunteer Army (PVA) crossed the Yalu and entered the war. The surprise Chinese intervention triggered a retreat of UN forces and Chinese forces were in South Korea by late December.

In these and subsequent battles, Seoul was captured four times, and communist forces were pushed back to positions around the 38th parallel, close to where the war started. After this the front stabilized and the last two years of fighting became a war of attrition, the war in the air, however, was never a stalemate. North Korea was subject to a massive U.S. bombing campaign. Jet fighters confronted each other in air-to-air combat for the first time in history, and Soviet pilots covertly flew in defense of their communist allies.

The fighting ended on 27 July 1953, when the Korean Armistice Agreement was signed. The agreement created the Korean Demilitarized Zone (DMZ) to separate North and South Korea, and allowed the return of prisoners. However, no peace treaty was ever signed, and the two Koreas are technically still at war, engaged in a frozen conflict. In April 2018, the leaders of North and South Korea met at the DMZ and agreed to work towards a treaty to formally end the Korean War.